The invention relates to light sources in combination with reflective multilayer films.
Many applications require polarized light to function properly. One example of such an application are optical displays, such as liquid crystal displays (LCDs), which are widely used for lap-top computers, hand-held calculators, digital watches, automobile dashboard displays and the like. Another such application is task lighting configurations for increased contrast and glare reduction.
To produce polarized light, a light source is typically coupled with one or more absorptive polarizers. These polarizers use dichroic dyes which transmit light of one polarization orientation more strongly than the orthogonal polarization orientation. Dichroic polarizers are very inefficient, however, in that light of the orthogonal polarization is largely absorbed and is therefore unavailable for application illumination. For example, a typical dichroic polarizer transmits only about 35-45% of the incident light emitted by a standard display backlight. This inefficiency is a major disadavantage to dichroic polarizers, as the light which is absorbed is not available for the associated application. In an LCD display, for example, the absorbed light does not contribute to the illumination, and thus the overall brightness, of the LCD.
Vacuum deposited, thin film dielectric polarizers are not absorbing, as are dichroic polarizers, but do suffer other disadvantages, such as poor angular response and poor spectral transmission for non-designed wavelengths. In addition, they are conventionally coated onto stable substrates, such as bulk optical glass or polymer substrates, which render them too bulky and heavy for use in applications requiring light weight and small profile.
Current technology, for LCD illumination, makes no attempt at polarization control other than use of inefficient, dichroic polarizers. Current technology, for glare reduction in task lighting and vehicle displays, does not use polarizers at all due to the inefficiencies of dichroics, and the bulk and angle performance of vacuum deposited dielectric polarizers.
The present polarized light sources described herein include a diffuse light source and a reflective polarizing element placed proximate thereto. The reflective polarizing element transmits light of a desired polarization and reflects light of another polarization back into the diffuse source. The light of the rejected polarization is reflected back into the diffuse source and is randomized. Some of the initially rejected light is thus converted into the desired polarization and is transmitted through the reflective polarizing element. This process continues, and the repeated reflections and subsequent randomization of light of the undesired polarization increases the amount of light of the desired polarization that is emitted by the polarized light source.
The diffuse source consists of a light emitting region and a light reflecting, scattering and depolarizing region. The source may be a fluorescent lamp, incandescent lamp, solid-state source or electroluminescent (EL) light source.
The reflective polarizing element may be a tilted dielectric film coated on a glass substrate, bulk optic or structured surface. The reflective polarizing element may also be a multilayered, birefringent polymeric film.
In a typical application, the polarized light source is used to illuminate an optical display, such as a Liquid Crystal Display (LCD). For this purpose, the polarized light source is used in combination with a means for delivering the polarized light to an optical display. This can include free space propagation, a lens system, or a polarization preserving light guide.
The polarized light source may also be used in various task lighting configurations such as vehicle dashboard displays, or fluorescent fixtures in an office lighting environment. For glare reduction, the polarized light source is placed to provide illumination to a specific task location where a manual or visual task is being performed.